Abstract

Soils across the grains cropping regions of Western Australia were inherently low in P and other nutrients. Development of agriculture would not have been possible without the use of P and other fertilisers. In these soils, profitable rates of P resulted in increased yields and a gradual build-up of soil available P (measured as Colwell P). Once P deficiency is corrected and the Colwell P values for near maximum crop production (critical values) are reached, the recommended practice is to maintain the soil at these critical values. This prevents reversion to deficiency and risk of yield and profit loss. The P maintenance practice uses less P than the build-up phase. The amounts of P applied during maintenance are designed to balance (1) removal in harvested grains and other products such as hay and sheep sold off farm (2) amounts of fertiliser P that becomes tied-up by strong adsorption with soil and (3) unavoidable losses due to leaching, runoff and erosion.

The WA cropping industry has a long history of P use that started with the development of the industry. It is now time to assess if we have succeeded in correcting the P deficiency and if we are ready to move to the maintenance phase. This is also an opportunity to assess what other soil constraints are limiting production and profits so that money freed-up by transitioning from build-up to maintenance could be re-invested in managing these constraints.